DEP5313-FIBER OPTIC COMMUNICATION SYSTEM TOPIC 2: COMPONENTS IN FIBER OPTIC COMMUNICATION SYSTEM TOPIC 2 COMPONENTS IN FIBER OPTIC COMMUNICATION SYSTEM (08 : 10) LEARNING OUTCOME At the end of learning session, students should understand 2.1 Optical devices in fiber optic systems 2.2 Types of Fiber optic connections 2.3 Multiplexing and demultiplexing techniques n fiber optic communication Block Diagram of Fiber Optic Communications Pulses Information Input (Voice or video) Coder and Converter Light Source Transmitter Digital data from computer Pulses Shaper Photocell or light detector Decoder Amplifier Digital data to computer Original voice or video Block Diagram Function TRANSMITTER SECTION (1) CODER/CONVERTER: • It is a ADC (analog to digital converter). • At the input , the Coder converts analog signal(analog information such as voice r video or computer data) into digital signals. • If the input signals are computer signals, they are directly connected to light source transmitter circuit. (2) LIGHT SOURCE • Is generally a FOCUS type LED (Light Emitting Diode) or low intensity laser beam source (such as Injection Laser Diode-solid state laser) or in some cases an infrared beam of light • The frequency of digital pulses control the rate, at which light source turns ON/OFF, in other word, this is how the digital signals are converted into equivalent light pulses and focused at one end of fiber-optic cable. Block Diagram Function (c) Fiber–optic cable • Fiber optic cable passes the light pulses that are fed to one end of fiber-optic cable on to the other end. • The cable has VERY LESS attenuation (loss due to absorption of light waves) over a long distance. • Its bandwidth is large; hence, its information carrying capacity is high. Block Diagram Function (1)RECEIVER SECTION - - LIGHT • DETECTOR or photodetector is tranducer that detect the light pulses and then converts it into proportional electrical signal. signal into analog signals, such as voice, video or computer data. • The electrical signals are then amplified • and reshaped into original digital pulses by the shaper • If the input signals are computer signals, the signal can be directly taken out from the output of the shaper circuit. (1) DECODER • It is a ADC (analog to digital converter). • Converts digital signal into analog signals, such as voice, video or computer data. Pulses before shaper process Pulses after shaper process 5V 0V Optical Transmitter • • • The transmitter consists of a lightsource and its drive circuitry. The light sources used for fiber optic transmitters need to meet several criteria: It has to be at the correct wavelength Be able to be modulated fast enough to transmit data Be efficiency coupled into fiber Two devices commonly used to generate light for fiber optic communication system: (a) Light emitting diode (LED) (b) LASER diode (LD) (a) Light Emitting Diode (LED) • An LED is a PN junction diode that is operated with forward bias. • Combination of electron and holes in depletion region generates photons of light: Photons are allowed to direction normal to the junction, the diode is called surface-emitting LED. If parallel to the junction, the diode is called edge-emitting LED. • LEDs can be visible spectrum or infrared. (a) Light Emitting Diode (LED) Characteristics • Light generation - emits light by spontaneous emission. • Have much lower power outputs than lasers. • Property of light - is an incoherent light source that emits light in a disorderly way(No internal order) • Transmission wavelength within 660-1650 nm. Typically used at 850nm and 1310nm. • Diverging light output pattern makes them harder to couple into fibers, thus limiting them to use with multimode fibers. • Have moderate bandwidth - Limited to systems operating up to about 250 MHz or around 10-100 Mbps and shorter distance multimode systems • Have a very broad spectral output (40-190nm) which causes them to suffer chromatic dispersion in fiber. LASER Diode (LD) Laser diodes are sometimes referred to as injection laser diodes or by the acronyms LD or ILD. • Laser diode are more complex than LED, although the basic mechanism is still forward-biased PN junction diode. However, the simplest diode lasers are structurally similar to LEDs. • Both generate light from recombination of electron hole pairs at a forward-biased junction but laser diode operates at higher current levels. • LD Characteristics • A laser diode emits light through stimulated emission rather than spontaneous emission, which results in higher output power. • Laser is a coherent light source that emits light in a very orderly way. • Relatively directional light output pattern makes them easily couple to single mode or multimode fibers. • A laser diode has a narrower an emission line width (spectral width) from 0.00001 to 10 nm, compared to common LED • Transmission wavelength within 780-1650 nm. Primarilly used at 1310nm and 1550nm. • High bandwidth capability, most being useful to well over 10 GHz and faster data transmission speed about 10 Gbps. Thus Ideal for long distance high speed links. • More expensive- creating the laser cavity inside the device is more difficult, the chip must be separated from the semiconductor wafer and each end coated before the laser can even be tested to see if its good. LED & LD Wavelengths The emission wavelength depends on the chemical composition of the diode. Material Wavelength range nm GaAs GaAIAs InGaAs InGaAsP 750-900 800-900 1000-1300 900-1700 Communications LEDs are most commonly made from GaAsP (1300 nm) or GaAs (810-870 nm) Most Laser Diodes emit in the near-infrared spectral region, but others can emit visible (particularly red or blue) light or mid-infrared light. The most common semiconductors used in laser diodes are compounds based on: Gallium Arsenide, GaAs - 750 to 900 nm in the infrared Indium Gallium Arsenide Phosphide, InGaAsP -1200 to 1700 nm in the infrared Gallium Nitride - near 400 nm in the blue. Wavelength for Different Colors Color Wavelength (nm) Red 780 - 622 Orange 622 - 597 Yellow 597 - 577 Green 577 - 492 Blue 492 - 455 Violet 455 - 390 Optical receiver (Light Detector) The main function of the receiver is to convert optical signal into electrical signal. An optical receiver consists of: photo diode semiconductor (photodetector) which produces current in response to incident light an amplifier signal conditioning circuitry Photo diode • Fiber optic receivers use two types of photo diodes: positive-intrinsic-negative (PIN) photo diode avalanche photo diodes (APD). • A junction photodiode is an intrinsic device that behaves similarly to an ordinary signal diode, but it generates a photocurrent when light is absorbed in the depleted region of the junction semiconductor • In a photodiode, a reverse bias potential is applied across the diode, preventing current from flowing in the absence of light. However, when expose to light, electron-hole pairs are created, generating a current. PIN photodiode consists of a thick doped intrinsic layer sandwiched between thin p and n regions. The major feature of a p-i-n PD is that its intrinsic layer is its depletion layer, where the absorption of photons occurs. Avalanche Photo Diode (APD) The APD photodiode structure is relatively similar to PIN photodiode structure. • APD internally amplifies the photocurrent by an avalanche process. • A large reverse-bias voltage (typically over 100 volts) is applied across the active region that will causes electrons to collide with other electrons in the semiconductor material. This process is called avalanche multiplication, and fraction of the electrons part of the photocurrent. • Photodetector Characteristics • Since the optical signal generally weakened and distorted when it emerges from the end of the fiber, the photodetector must meet strict performance requirements such as: A high sensitivity to the emission wavelength range of the received light signal A minimum addition of noise to the signal A fast response speed to handle the desired data rate • Sensitivity measures the response to an optical input signal as a function of its intensity. Photodetector’s sensitivity can be measured in two concepts: a) quantum efficiency b) responsivity. Photodetector Characteristics a) Quantum efficiency , η measures the fraction of incoming photons that generate electrons at the detector. b) Responsivity, ρ is the ratio of current output (photo current) to light input. where λ0 is measured in um (micrometers) η is the quantum efficiency It is defined as: High responsivity equals high receiver sensitivity. • Since in fiber optic communication systems, input powers are usually in microwatt level, responsivity is often expressed as µA/µW. • Photodetector Characteristics Speed of Response The speed of response and bandwidth of a photodetector depend on three factors: The transit time of the photo-generated carriers through the depletion region The electrical frequency response as determined by the RC time constant, which depends on the diode’s capacitance The slow diffusion of carriers generated outside the depletion region Spectral Response The wavelength that a photo-detector can respond to depends on its composition. • The following graph shows the detector response curve for different materials. • Photodetector Characteristics Dark Current • is the current through the photodiode in the absence of light, when it is operated in photoconductive mode. • Is the baseline noise current developed by the random generation of electrons and holes within the depletion region of a photodiode, without the addition of an external bias current or light activation • The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. • Dark current sets a floor on the minimum detectable signal, because a signal must produce more current than the dark current in order to be detected. • Dark current is also a source of noise when a photodiode is used in an optical communication system. Typical Performance Characteristics of Detectors Silicon Germanium InGaAs Parameter PIN Wavelength range (nm) APD PIN 400 – 1100 APD PIN 800 – 1800 APD 900 – 1700 Peak (nm) 900 830 1550 1300 1300 (1550) 1300 (1550) Responsivity ρ (A/W) 0.6 77-130 0.65-0.7 3-28 0.63-0.8 (0.750.97) Quantum Efficiency (%) 65 – 90 77 50-55 55-75 60-70 60-70 Gain (M) 1 150-250 1 5-40 1 10-30 Excess Noise Factor (x) - 0.3-0.5 - 0.95-1 - 0.7 Bias Voltage (-V) 45-100 220 6-10 20-35 5 <30 Dark Current (nA) 1-10 0.1-1.0 50-500 10-500 1-20 1-5 Rise Time (ns) 0.5-1 0.1-2 0.1-0.5 0.5-0.8 0.06-0.5 0.1-0.5 Noise factor Noise is unwanted components of the signal that tend to disturb the transmission and processing of the signal in a physical system. Noise generated by the photodiode is most critical. The three most predominant types: 1) Thermal Noise- A noise due to the random motion of electrons or dissipation of heat in the detector resistance. 2) Shot Noise - is a small current produced from the randomness of the photon-to-electron conversion. 3) dark current Noise - is a very small current present when no light is incident on the photodetector SIGNAL-TO-NOISE RATIO SNR The ratio of the total signal to the total noise shows how much higher the signal level is than the level of the noise. It is a measure of signal quality. The signal-to-noise ratio, SNR (or S/N) at the output of an optical receiver is defined as the ratio between the signal power and the noise power and presented as follow: where: i2noise = it2 + is2 + id2 RECAP Tutorial 1 • A Si PIN photodiode is operating at 50 GHz at 300K. The current is 200 µA, the dark current is 0.5 nA and the load resistance is 50 M ohm. Find the thermal noise, shot noise, dark current noise and total noise Tutorial 2 • The Si PIN photodiode in Exercise 1 has an incident power of 417 µW and a responsivity of 0.48. Find the SNR. Tutorial 3 • Suppose we have a system consisting of an LED emitting 10mW at 0.85µm, a fiber cable with -20 dB of loss, and a PIN photodetector of responsivity(ρ) 0.5A/W. The detector’s dark current is 2 nA. The load resistance is 50Ω; the receiver’s bandwidth is 10MHz, and its temperature is 300K (27oC). The system losses, in addition to the fiber attenuation, include a -14 db power reduction due to source coupling and a -10dB loss caused by various splices and connectors. • Compute the I. received optic power, II. the detected signal current and power, III. the shot noise and thermal noise, and IV. the signal to noise ratio Solution The total system loss is (-20) + (-10) + (-14) = -44dB. We know loss 10 log10 x = -44dB So, transmission efficiency of 10-4.4 = 4 x 10-5. i. The optic power reaching the receiver is then PR = 4 x 10-5(10) = 4 x 10-4mW = 0.4 µW ii. Detected signal current / photocurrent = 0.5 (0.4) = 0.2µA = 200nA Solution The dark current only 2nA is small compared to the signal current, so it can be ignored in this example. The electrical signal power is PES = (0.2 x 10-6)2 (50) = 2 x 10-12W = 2(1.6x10-19) (0.2x10-6)(107)(50) = 3.2 x 10-17W Thermal Noise power PNT = 4 (1.38 x 10-23) (300) (107) = 1.66 x 10-13W In this system, the thermal noise is nearly four orders magnitude greater than the shot noise. The thermal noise limited result applies. We can compute the SNR from the equation CONNECTION IN FIBER OPTIC Fiber optic cable is terminated in two ways : 1) with connectors that can mate two fibers to create a temporary joint and/or connect the fiber to a piece of network gear 2) with splices which create a permanent joint between the two fibers. (1) CONNECTOR An optical fiber connector terminates the end of an optical fiber, and enables quicker connection and disconnection than splicing. The connectors mechanically couple and align the cores of fibers so that light can pass. Good connectors lose very little light due to reflection or misalignment of the fibers. (1) CONNECTOR Type CHARACTERISTICS • available in single mode and ST multimode. Straight Tip • simplex only, twist-on mechanism. •simplex only, screw-on FC mechanism. Ferrule • available in single mode Connector and multimode LC Lucent/ Local Connector •simplex and duplex, push and latch •available in single mode and multimode Connector Adapter/Coupler (1) CONNECTOR Type Characteristics SC Subscriber • Connector • simplex and duplex, snap-in mechanism. available in single mode and multimode. FDDI • 2.5mm ferrules connectors • duplex multimode . connector generally used to connect to the equipment from a wall outlet, but the rest of the network will have ST or SC Connector Adapter/Coupler (2) SPLICING Fiber splicing is the process of permanently joining two fibers together. There are two types of splices: a) fusion b) mechanical (a) Fusion Splicing In fusion splicing, two fibers are literally welded (fused) together by an electric arc. is done by an automatic machine called fusion splicer, which mechanically aligns the two fiber ends, then applies a spark across the fiber tips to fuse them together. Fusion splicing is the most widely used method of splicing as it provides lowest insertion loss and virtually no back reflection. (a) Fusion Splicing generates spark (high temperature heat) • Fusion arc in Splice complete (b) Mechanical Splicing Mechanical splicing uses mechanical fixtures to join two fibers together end to end. Mechanical splicing join two fiber ends either : by clamping them within a structure by gluing them together is include transparent adhesives and index matching gels. Transparent adhesives are epoxy resins that seal mechanical splices and provide index matching between the connected fibers. Types of mechanical splices The biggest difference between mechanical splices is the way the fibers are aligned. Some types of mechanical splices include • capillary type • V-groove and rotary devices. • plastic, glass, metal, • ceramic tubes Types of mechanical splices Capillary type is the simplest method of making a mechanical splice. Two fibers are inserted into a thin capillary tube. The tube has a inner diameter that matches the fiber's cladding diameter. (The fibers must first have coatings removed and cladding exposed and cleaned). These two fiber ends are pushed inwards until they meet. Index matching gels are often inserted in the center to reduce back reflections. Types of mechanical splices Ribbon V- Groove type V-groove splices are quite simple and work well for single fiber or even for fiber ribbons. For ribbon fibers, capillary type doesn't work anymore. Instead, fiber ribbon is put in a V-shaped groove array, with each fiber place in its own v-groove. Two ribbon fibers are butted together in this V-groove array and then a cover plate is applied on top. This method is primarily used for splicing a multi-fiber cable in a single action. Types of mechanical splices Elastomeric type 35 Elastomeric splice is for lab testing or emergency fiber repairs. Same as V-groove type, it has a single fiber v-groove but the v-groove is made of flexible plastic. First an index matching gel is injected into the hole, then one fiber is inserted until it reaches about halfway. The other fiber is then inserted from the other end until it meet the first one. Splices, from left: FUSION SPLICE, ELASTOMERIC, ULTRASPLICE (capillary splice), camlock, FIBERLOK (V-Groove type), T&T Rotary Splice Fusion vs Mechanical splicing Characteristic Arc Fusion Mechanical Fiber alignment mechanism machine is used to precisely align the two fiber ends then the glass ends are "fused" or "welded" together using some type of heat or electric arc. simple alignment devices to hold the two fiber ends in an alignment fixture with a transparent gel or optical adhesive. Loss and back reflection lower loss (Typical loss: 0.1 higher loss (Typical loss: dB )and less back reflection 0.3 dB) and greater reflectance Fiber types are used primarily with single mode fiber What else? work with both single and multi mode fiber. REFERENCES Agrawal, Govind P. (2010). Fiber-Optic Communication Systems. (Fourth Edition). Wiley Series. (ISBN : 978-0-47050511-3). Downing , James N. (2005). Fiber-Optic Communications, Thomson Delmar Learning. (ISBN: 1-4018-6635-2). George Kennedy, Bernard Davis. (2006). Electronics Communication Systems.(4th). McGraw Hill. Jim Hayes, (2010). Fiber Optics. Technician’s Manual, Fourth Edition. Thomson Delmar Learning. Joseph C. Palais, (2005) Fiber Optic Communications. Fifth Edition. Pearson / Prentice Hall. (ISBN 0130085103, 9780130085103).